The present invention relates generally to a mold used to manufacture a member formed through metal mold casting, and a manufacturing method of the member. The present invention also relates to a damper manufacturing mold, and more particularly to a damper manufacturing method that uses metal mold casting. The present invention is suitable, for example, for a manufacture of a damper used for a hard disc drive (“HDD”). Although a description will now be given of a damper as the member for the HDD, the present invention is also applicable to another member.
Along with the recent widespread Internet, etc., providing of an inexpensive magnetic disc drive that records a large amount of information including still and motion pictures is increasingly demanded. A high recording density disc drive needs high-accuracy head positioning performance, and it is necessary for this purpose to precisely produce a housing that houses a recording medium with reduced vibrations and deformations. An environmentally friendly characteristic is also important, such as a reduced noise during operations and an effective use of a material in the manufacturing process.
Aluminum die casting is used to produce a housing of a magnetic disc drive for a precise shape so as to use the same coefficient of linear expansion as that of a head actuator, etc. In addition, the housing is made heavier to reduce the noises and the vibrations by attaching a weight (vibration damper) to the housing and by damping the vibrational energy. See, for example, Japanese Patent Application, Publication No. 2002-124072.
Since aluminum has such a low specific gravity as 2.7, a damper is made of a material, such as iron having such a high specific gravity as 7.9, stainless steel having a specific gravity of 7.9, and brass having a specific gravity of 8.3.
Use of brass for the damper is advantageous since it has a high specific gravity. However, its material cost is high, and it needs a corrosion protection treatment. Iron is inferior to brass in workability, but the material cost is lower. However, in comparison with brass, iron expensively requires a thicker corrosion protection treatment. On the other hand, stainless steel does not need a corrosion protection treatment, but its material cost is high and inferior in workability.
A conceivable damper manufacturing process with a metallic material includes stamping using a pressing machine, lost wax process, metal mold casting (including die casting), and so on.
The magnetic disc drive needs to arrange a damper in the housing, but the housing has only a limited attachment space. Therefore, a comparatively thick damper is needed to increase the damper weight. As for a stamping process, a large size and increased thickness would increase a stamping machine size and its comparatively large material margin causes a large material loss and the cost increase. In addition, it is difficult for stamping to form a three-dimensional shape that has a thick part and a thin part on the one-body member.
Lost wax process is a self-extinguishing casting that destroys a die whenever the damper is manufactured, complex and takes a long process, and therefore expensive. Forging, machining, sintering, resin injection molding, etc. are also conceivable but these methods have merits and demerits, such as a large working facility, a long cycle time, and a high material cost.
FIG. 6 shows a table that indicates a relationship among a manufacturing process, a high specific gravity material, and a cost ratio. FIG. 6 also shows metal mold casting, in addition to the above material and manufacturing process. As understood from FIG. 6, the metal mold casting (labeled as “zinc die casting” in FIG. 6) is well-balanced among the candidate manufacturing processes in view of the material cost and productivity, and most economical in view of the cost ratio of 0.15.
Metal mold casting is a casting method by filling molten metal in a durable mold, and a method for mass-producing precise cast with fine casting surfaces by filling the molten metal in a durable die is particularly referred to as die casting. Metal mold casting generally has a filling process, a gate cutting process, and a surface treatment process. The filling process fills molten metal from an injection gate in a mold, cools down and solidifies it. The cutting process cuts the gate from a cast. The surface treatment process is a painting or plating so as to prevent a surface corrosion and micro-dust from the casting surface.
Prior art include, for example, Japanese Patent Applications, Publication Nos. 2005-313220 and 2002-124072. Saburo Kobayashi, “Design and Manufacture of Die Cast Die,” FIG. 3.1, Nikkan Kogyo Shinbun, Ltd., Dec. 24, 1993.
High specific gravity metal suitable for metal mold casting is brass and zinc alloy. Brass has a specific gravity higher than zinc alloy but its melting point is so high that the durability is low and unsuitable for the mass production. Accordingly, the instant inventors have studied metal mold casting that utilizes zinc alloy.
In this respect, Japanese Patent Application, Publication No. 2002-124072 discloses, at paragraph no. 0019, that “each of Cu, Zn, and stainless steel has a specific gravity higher than aluminum of the base 12, and is suitable for a material of a damper 42. In addition, when the damper 42 is made of a metallic material, it can be easily manufactured through a well-known process, such as casting and forging. Casting and forging can manufacture the damper 42 integrally.” Therefore, this reference discloses a concept of manufacturing a damper through metal mold casting of zinc (Zn).
However, in reality, when a prototype of a damper is made of zinc alloy (ZDC2) through metal mold casting, generated gas from the molten metal and enclosed gas during molten metal filling process causes surface defects (rough casting surface), and the subsequent surface treatment suffers adhesion defects and peeling of painting and plating. The surface treatment intends to prevent micro-dust from the casting surface as well as corrosion of zinc alloy. The surface-treated damper is fixed onto the backside of the housing of the disc drive. Thereafter, the housing is put in a clean room, and mounted with a disc, a carriage, etc. The degraded surface treatment, such as an exposure of casting surface and peeling due to adhesion defects, may contaminate the disc drive and the clean room, and resultantly affect the manufacturing quality of the disc drive.
Avoiding the above problems, as shown in FIG. 7, Kobayashi proposes in FIG. 3.1 to provide a overflow gate connected to an overflow well to the entire perimeter of the mold except an injecting side from which molten metal is introduced from a biscuit or casting port (i.e., three sides in this case) so as to remove generated gas from the molten metal filled in the box-shaped mold (with no lid).
However, the overflow gate provided to the entire perimeter of the mold as proposed by Kobayashi in manufacturing the damper increases a material waste, takes time to fill the molten metal in the mold, and causes porosity and misrum. In addition, Kobayashi increases the cutting force in the cutting process that separates gates from a cast, a size of the cutting machine, and the cost. It is thus preferable to limit the overflow gate to a position where the stagnant gas is likely to occur. The place where the stagnant gas is likely to occur depends upon the convection flow of molten metal.